Dipolar organic materials producing highly efficient laser-like emission

ABSTRACT

High-efficiency laser-like emission at low thresholds in dipolar organic materials upon pulsed optical excitation, without using any external mirrors. Unusually high conversion efficiencies and low thresholds in laser-like emission have been observed in the solutions of organic molecular salts having large dipole moments and specific dye molecules having high photoluminescence efficiencies. Pumped with frequency-doubled pulses from a Nd:YAG laser, conversion efficiencies in the range of 15-40% were achieved without incorporation of external mirrors. The threshold pump energies for such emission have been observed to be low (&lt;8 μJ). The spectrally narrowed output beam was found to have low divergence, high degree of polarization, and pulse-width less than that of the excitation pulses (50 picoseconds). The exceptionally low threshold (&lt;1 μJ) and high energy conversion efficiencies observed in molecular salts have been attributed to the large excited-state dipole moment of these noncentrosymmetric molecules favoring strong cooperative (laser-like) emission in spite of small photoluminescence quantum efficiencies.

FIELD OF THE INVENTION

[0001] The present invention relates generally to the sources ofelectromagnetic energy, and in particular, to the strong cooperativealignment in dipolar organic media upon pulsed optical excitationresulting in highly efficient mirrorless laser-like emission.

BACKGROUND OF THE INVENTION

[0002] There has been considerable interest in mirrorless spectrallynarrowed laser-like emission from organic dyes and conjugated polymers.An early observation of such laserlike emission occurring in organicdyes was made by Mack (Appl. Phys. Lett. 15, 166 (1969)). Usingshort-pulse optical excitation of several polymethine cyanine dyes(cryptocyanine, 1′,1-diethyl-2,2′-dicarbocyanine iodide (DDI), and3,3′-diethylthiatricarbocyanine iodide (DTTC)), Mack had obtainedspectrally narrowed directional emission having typical linewidth of13-18 nm with an energy conversion efficiency of up to about 3 percent.Following this work, several other groups reported experimental (U.Ganiel, A. Hardy, G. Neumann, and D. Treves, IEEE J. Quantum Elec.QE-11, 881 (1975), C. S. Wang, W. H. Cheng, C. J. Hwang, W. K. Burns,and R. P. Moeller, Appl. Phys. Lett. 41, 587 (1982)) and theoretical (L.Allen and G. I. Peters, Phys. Lett. 31A, 95 (1970), L. W. Casperson andA. Yariv, IEEE J. Quantum Elec. QE-8, 80 (1972), L. W. Casperson, J.Appl. Phys. 48, 256 (1977)) investigation of spectral narrowing withoutfeedback of the emitted radiation.

[0003] Glessner et al. (J. Appl. Phys. 62, 5 (1987)) reported amplifiedspontaneous emission (ASE) in the spectral range of 1.0-1.34 micronsfrom an iodine filled cell utilizing a YAG laser pumped dye laser systemas the excitation source, achieving about 1% conversion efficiency.However, later they achieved conversion efficiency of 8.3% at anelevated temperature and pressure conditions (U.S. Pat. No. 4,905,247(1990)).

[0004] Significant advancement has been made in recent years inspectrally narrowed light emission using solid films of conjugatedpolymers. Frolov et al. reported mirrorless lasing in thin films ofpoly(2,5-dioctyloxy-p-phenylenevinylene) (DOO-PPV), which theyidentified as superradiance (Jpn. J. Appl. Phys. 35, L1371 (1996)), oremission due to the formation of local cavities via scattering (OSAAnnual Meeting, Paper ThG5, Santa Clara, Sep. 26-30 (1999)). Laser-likeemission from such solid films usually can not continue beyond a maximumof 10,000 shots.

[0005] Hide et al. observed low gain narrowing threshold (1 μJ perpulse) in sub-micrometer thick films of semiconducting polymers based onwaveguide structures (Science 273, 1833 (1996)). Superradiant emissionin neat films of an alternating copolymerpoly[dimethylsilylene-p-phenylenevinylene-(2,5-di-n-octyl-p-phenylene)-vinylene-p-phenylene](Si-PPV) was observed by Brouwer et al. (Adv. Mater. 8, 935 (1996)).Spectral narrowing in optically pumped poly(p-phenylenevinylene) (PPV)films was reported by Denton et al. (Adv. Mater. 9, 547 (1997)).

[0006] Laser action was also observed in solutions of several conjugatedmaterials placed in conventional resonator cavities (D. Moses, Appl.Phys. Lett. 60, 3215 (1992) and U.S. Pat. No. 5,237,582 (1993), H.Brouwer, V. V. Krasnikov, A. Hilberer, J. Wildeman, Appl. Phys. Lett.66, 3404 (1995), W. Holzer, A. Penzkofer, S. Gong, A. Bleyer, and D.Bradley, Adv. Mater. 8, 974 (1996)).

[0007] Two-photon pumped up converted lasing has been demonstrated indye doped polymer waveguides (A. Mukherjee, Appl. Phys. Lett. 62, 3423(1993)), and solid matrices (He et al., IEEE J. Quantum Elec. 32, 749(1996), Prasad et al., U.S. Pat. No. 5,912,257 (1999)). The presentinvention relates to laser-like emission using single-photon excitation.

[0008] In terms of applications, the major weaknesses of mirrorlesslaser-like emission have been the small conversion efficiency (<8%) andhigh pump energy thresholds (>10 μJ). Applications have not materializedbecause of these weaknesses. Clearly, appropriate materials that canlead to high conversion efficiencies (>15%) and lower thresholds (<5microjoules) in mirrorless lasing are highly desired. Such a devicewould provide a low-cost source of short-pulse (picosecond) laser-likeemission for various commercial and research related applications. Thesemirrorless devices would not require expensive optical elements,alignment accessories, and the extensive time needed for alignment andthus would substantially reduce the cost. The fact that dipolarstructure may result in large conversion efficiencies and require lowthresholds were not identified and reported in the literature so far.The present invention involves the invention that dipolar moleculeswhich photoluminesce produce laser-like emission with exceptionally highconversion efficiency and at low pump-energy thresholds.

SUMMARY OF THE INVENTION

[0009] The present invention demonstrates mirrorless laser-like emissionwith exceptionally large conversion efficiencies at low thresholds usingdipolar organic materials. Strong cooperative emission without aresonant cavity is due to the macroscopic dipoles arising from coherentinteractions of the photo-excited species in the presence of an intenseoptical field. Hence strongly dipolar molecules should lead to a moreextensive order in the dipole moments (or phasing), leading to asignificantly enhanced efficiency in cooperative emission. It has beendiscovered that organic molecular salts having large ground-state andexcited-state dipole moments are excellent candidates for mirrorlesslaser applications despite very small PL efficiencies. High-efficiencyemission in the mirrorless configuration in organic dyes having moderatedipole moments but large PL efficiencies has also been observed.

[0010] Highly efficient directional emission in the spectral range of600-620 nm is demonstrated upon excitation of solutions of the organicsalts, such as, styrylpyridinium cyanine dye (SPCD),4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST),4′-diethylamino-N-methyl-4-stilbazolium tosylate (DEST), and4′-dimethylamino-N-methyl-4-stilbazolium iodide (DASPI), bysecond-harmonic pulses from a Nd:YAG laser. Large electro-opticcoefficients were previously measured in the first two of these saltsorganized into noncentrosymmetric single-crystals (M. Thakur, J. Xu, A.Bhowmik, and L. Zhou, Appl. Phys. Lett. 74, 635 (1999), T. Yoshimura, J.Appl. Phys. 62, 2028 (1987), A. K. Bhowmik, A. Mishra, S. Sodah, and M.Thakur, Bulletin of Am. Phys. Soc. 44, 1431 (1999)). The dyes with highPL efficiencies that have been investigated for mirrorless laser-likeemission include rhodamine 6G and DCM.

[0011] The threshold excitation pulse-energy for laser-like emissionfrom the strongly dipolar salts is less than 1 μJ per pulse, while theenergy conversion efficiency in the range of 20-40% has been obtaineddespite a PL efficiency of only ˜0.3%. The threshold for mirrorlesslasing in the highly luminescent dipolar dyes are slightly larger andthe energy conversion efficiencies somewhat smaller than that for thesalts.

[0012] Highly directional output beam was obtained from the mirrorlesslasers, with the half-apex angle of divergence lass than 10 mrad. Theoutput emission was frequency-doubled using a commercially availabletype-I phase-matched β-barium borate (BBO) crystal to generate 305-310nm radiation. The pulse duration was measured by background-freesecond-harmonic generation (SHG) intensity autocorrelation technique tobe ˜32 ps, while the excitation pulsewidth was ˜80 ps. Thus, the presentinvention provides a cost-effective frequency shifter for short laserpulses without requiring high-precision optical components.

[0013] Mirrorless solid-state lasing devices are also disclosed whichare constructed using the dipolar organic salts doped into solidmatrices of poly(methyl methacrylate) (PMMA). Spectrally narroweddirectional emission with typical bandwidth of ˜10 nm is achieved.

[0014] The present invention has the following characteristics:

[0015] 1. A dipolar organic material producing highly efficientlaser-like emission at low thresholds without external mirrors.

[0016] 2. A highly efficient and low-threshold mirrorless lasers(producing laser-like emission without mirrors) comprising:

[0017] (a) organic materials producing highly efficient laser-likeemission at low thresholds without external mirrors in solution asactive media; and

[0018] (b) a pump laser projecting the excitation beam into the activemedia.

[0019] 3. A mirrorless laser of claim 2, comprising organic moleculeshaving large dipole moments as active media.

[0020] 4. A mirrorless laser of claim 2, comprising dipolar organic dyeshaving large photoluminescence efficiencies as the active media.

[0021] 5. A mirrorless laser of claim 3, comprising strongly dipolarorganic molecular salts having the following chemical formula as theactive media:

[0022] where R and R′ are the same or different, and comprise a moietyselected from the group consisting of alkyl, substituted alkyl, benzyl,and substituted benzyl, and Y⁻ is an anion.

[0023] 6. A mirrorless laser of claim 5, wherein both R and R′ are —CH₃,and Y⁻ is CH₃OSO₃ ⁻.

[0024] 7. A mirrorless laser of claim 5, wherein both R and R′ are —CH₃,and Y⁻ has the following formula:

[0025] 8. A mirrorless laser of claim 5, wherein both R and R′ are —CH₃,and Y⁻ is I⁻.

[0026] 9. A mirrorless laser of claim 5, wherein both R and R′ are—CH₂CH₃, and Y⁻ has the following formula:

[0027] 10. A mirrorless laser of claim 2 comprising dipolar organicdyes, such as rhodamine 6G (R6G) and DCM, having large photoluminescenceefficiencies as the active media.

[0028] 11. A mirrorless laser of claim 2 comprising a pump laser at awavelength where the active material has strong absorption.

[0029] 12. A mirrorless laser of claim 2 comprising a pump laseremitting optical pulses having pulsewidth shorter than the duration(about <100 picoseconds) of the excitation pulses.

[0030] 13. A mirrorless laser of claim 2 comprising a pump sourceproducing 1-100 picosecond pulses frequency-doubled by a nonlinearoptical crystal such as potassium dihydrogen phosphate (KDP).

[0031] 14. A mirrorless laser of claims 6-9 having threshold excitationpulse-energy less than about 1 microjoule with a line excitation ofabout 5 mm² area.

[0032] 15. A mirrorless laser of claim 2 yielding energy conversionefficiencies of more than about 15%.

[0033] 16. A mirrorless laser of claim 2 yielding energy conversionefficiencies of up to about 40%.

[0034] 17. A mirrorless ultraviolet short-pulse emitting laserconstructed by frequency-doubling the output of the laser of claim 2using a nonlinear optical crystal such as β-barium borate (BBO).

[0035] 18. A mirrorless laser emitting picosecond pulses at 300-310 nmconstructed by frequency-doubling the output of the lasers of claims6-9.

[0036] 19. The mirrorless lasers claimed above are stable undercontinuous operation for at least about 5 million shots.

[0037] 20. A mirrorless solid-state laser constructed using stronglydipolar organic molecules doped into solid polymeric matrices.

[0038] 21. A mirrorless solid-state laser of claim 15 constructed usingthe active materials of claims 4-7 doped into solid matrices ofpoly(methyl methacrylate) (PMMA).

[0039] Other objects, features, and advantages of the present inventionwill become apparent upon reading the following detailed description ofembodiments of the invention, when taken in conjunction with theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040] The invention is illustrated in the drawings in which likereference characters designate the same or similar parts throughout thefigures of which:

[0041]FIG. 1 shows the schematic diagram of the mirrorless lasing device(1), excitation source (2), variable attenuator (3), focusing system(4), cell containing the active material.

[0042]FIG. 2 illustrates the emission spectra of SPCD: a) low-energyphotoluminescence spectrum, and b) spectrum obtained with 10 μJexcitation pulse-energy. The excitation area was about 5 mm². The PL andnarrowed spectra have FWHM of 77 nm and 10 nm, respectively.

[0043]FIG. 3 illustrates the evolution of the spectral line-width ofemission from SPCD as a function of incident pulse energy. The thresholdexcitation pulse-energy for spectral narrowing is less than 1 μJ.

[0044]FIG. 4 shows the output pulse energy obtained from mirrorlesslasing in SPCD as a function of the energy of the pump pulses. The slopeefficiency beyond the threshold excitation energy is about 40%.

[0045]FIG. 5 shows the autocorrelation traces of the excitation pulsesfrom the Nd:YAG laser with pulsewidth ˜80 ps and the output pulses fromthe SPCD mirrorless laser with pulsewidth ˜32 ps.

[0046]FIG. 6 illustrates the emission spectra of DAST: a) low-energyphotoluminescence spectrum, and b) narrowed spectrum obtained with 15 μJexcitation pulse-energy.

[0047]FIG. 7 illustrates the emission spectra of DEST: a) low-energyphotoluminescence spectrum, and b) narrowed spectrum obtained with 15 μJexcitation pulse-energy.

[0048]FIG. 8 illustrates the emission spectra of DASPI: a) low-energyphotoluminescence spectrum, and b) narrowed spectrum obtained with 15 μJexcitation pulse-energy.

[0049]FIG. 9 illustrates the threshold input pulse energy for mirrorlesslasing in SPCD solution in methanol versus the concentration. Theminimum threshold pulse energy is <1 μJ at 0.1 mol/L.

[0050]FIG. 10 illustrates the emission spectra of a mirrorlesssolid-state laser constructed by doping SPCD in a PMMA matrix: a)low-energy photoluminescence spectrum, and b) narrowed spectrum obtainedwith 10 μJ excitation pulse-energy. The concentration of SPCD in PMMAwas 5×10⁻³ mol/L.

[0051]FIG. 11 shows a table of the chemical formula of the molecularsalts investigated.

[0052]FIG. 12 shows a table of the characteristics of the mirrorlesslaser-like emission from the molecular salts.

[0053]FIG. 13 shows a table of the comparison of the performances ofSPCD and a few other efficient laser dyes and conjugated materials.

DETAILED DESCRIPTION OF THE INVENTION

[0054] A schematic diagram of the mirrorless lasing device is shown inFIG. 1. The device comprises a main body containing the emissive activemedia, an appropriate focusing system, and an excitation source,typically a laser producing short optical pulses at a wavelength wherethe active material has strong absorption. The body containing theactive media could be a typical dye cell of convenient dimensions. Aquartz cuvette with square cross-section (1 cm×1 cm) was used. Afrequency-doubled Nd:YAG laser producing ˜80 ps pulses at 10 Hzrepetition rate at 532 nm wavelength was used as the source of theexcitation radiation. Pulses from the Nd:YAG laser, frequency-doubled bya potassium dihydrogen phosphate (KDP) crystal, were focused onto thedye cell using the combination of a spherical and a cylindrical lens toform a line excitation of ˜5 mm² area. The energy of the input pulseswas controlled using a variable attenuator. The energy of the input andoutput pulses from the Nd:YAG laser and the dye cell was measured usingpyroelectric detectors (DigiRad Model R-752 Laser Radiometer with P-444probe). An optical multichannel analyzer (ISA Model 270M) was used tomeasure the emission spectra collected at an angle normal to the pumpbeam as a function of the incident pulse-energy.

[0055] The characteristics of the output radiation from the devicecritically depends on the emissive material placed inside the cell. Theactive media used in conventional lasers must have very highphotoluminescence (PL) quantum efficiencies. In fact, the widely usedlaser dyes have PL efficiencies approaching unity. However, we havediscovered that strongly dipolar organic molecular materials providemirrorless spectrally narrowed laser-like emission with very largeefficiency at low threshold excitation energy despite, as will beelaborated, relatively low PL efficiencies. Several organic salts withlarge ground-state and excited-state dipole moments, listed in Table I,were investigated. Mirrorless laser-like emission has also been obtainedfrom highly luminescent dyes with moderate dipole moments, such asrhodamine 6G and DCM. Hence, the active material must possess eitherlarge dipole moment or high PL efficiency.

[0056] The compounds of the present invention can be generally stated asbeing:

[0057] where R and R′ are alkyl, substituted alkyl, benzyl, andsubstituted benzyl, and Y⁻ is an anion. The chemical formula of themolecular salts investigated is shown in FIG. 11.

[0058] The absorption spectrum of a dilute solution (1.36×10⁻⁵ mol/L) ofa typical salt SPCD in methanol, measured using a Hitachi U-2000spectrophotometer, showed that the absorption peak is at 480 nm. Theextinction coefficient, ε, was determined to be 5.43×10⁴ Lmol⁻¹cm⁻¹ atthe absorption peak, which corresponds to an absorption cross section,σ, of 2.07 Å².

[0059] A broad photoluminescence (PL) spectrum with full-width athalf-maximum (FWHM) of 77 nm centered at 620 nm was obtained with lowenergy excitation of the SPCD solution. Thus the Stokes shift is aslarge as 140 nm, which leads to negligible reabsorption of the emittedradiation. The FWHM of the emission spectra decreased significantly asthe energy of the excitation-pulse was increased. The thresholdpump-energy for spectral narrowing was observed to be less than 1 μJ,beyond which a gain-narrowed peak centered at 620 nm superseded the restof the emission band. At higher energies the tail of the PL bandcompletely disappeared. The emission spectra for incident pulse-energybeyond threshold was found to have a FWHM of 10 nm. FIG. 2 shows thelow-energy photoluminescence spectrum and the narrowed spectrum obtainedwith 10 μJ excitation pulses. The evolution of emission line-width as afunction of the pump energy is shown in FIG. 3.

[0060] The output laser-like beam beyond the threshold excitation energywas polarized and with a small divergence. The half-apex angle ofdivergence of the beam was estimated to be less than 10 mrad, indicatinghigh directionality. A measurement of the polarization of the beamindicated a ratio of intensities along and normal to the excitationpolarization direction to be approximately 3:1, whereas the PL emissionat lower energy was found to be unpolarized. The plot of the outputpulse energy obtained from mirrorless lasing in the 0.1 molar solutionof SPCD versus the input pulse energy is given in FIG. 4. It indicatesthat the threshold input energy for cooperative emission is indeed lessthan 1 μJ. The output energy from the dye cell increased linearly withthe increase in input pulse energy, as expected in the case of lasing.The pulse duration of the output radiation was measured by thebackground-free second-harmonic generation (SHG) intensityautocorrelation technique. A commercially available BBO crystal was usedto frequency-double the laser-like beam to generate radiation in thespectral range of 305-310 nm. The pulsewidth was measured to be ˜32picoseconds, while the duration of the excitation pulses was ˜80picoseconds. FIG. 5 shows the autocorrelation traces of the excitationpulses from the Nd:YAG laser and the output pulses from the mirrorlesslaser employing SPCD as the active material.

[0061] Output pulses of as high as 80 μJ energy were obtained frommirrorless laser-like emission from SPCD when pumped with 200 μJ inputpulses, indicating an energy conversion efficiency of 40%. Table IIlists the peak emission wavelength and conversion efficiencies obtainedusing several organic molecular salts. Very large efficiencies, up to40%, were achieved by using the molecular salts. The thresholdexcitation pulse-energy for spectrally narrowed laser-like emission fromall these strongly dipolar organic materials was measured to be lessthan 1 μJ. FIGS. 6, 7, and 8 illustrate the PL spectra and the narrowedemission spectra of DAST, DEST, and DASPI, respectively. The emissioncharacteristics of the mirrorless lasers constructed using the molecularsalts are given in Characteristics of the mirrorless laser-like emissionfrom the molecular salts are shown in FIG. 12.

[0062] The quantum efficiencies of photoluminescence (PL), φ, of thesolution of the dipolar salts in methanol were measured on aPerkin-Elmer LS-50B luminescence spectrometer. Using R6G as thestandard, the maximum φ of SPCD and DAST was estimated to be 3%. At theconcentration for which 40% conversion is achieved, is even lower(<0.3%). In spite of relatively poor PL efficiencies, cooperativeemission in these strongly dipolar organic materials yieldsexceptionally large conversion efficiencies beyond the thresholdexcitation pulse-energy. This demonstrates the role of molecular dipolemoment in the cooperative emission process. Organic salt moleculeshaving large dipole moments in the excited state leads to such uniqueresults. For a pump energy below the threshold the material behavessimilar to a typical dye molecule with a relatively low conversionefficiency. However, as pump energy or the optical field is increased,the excited-state dipoles and the total induced dipole moments form acorrelated or ordered state which decays to the ground state with asignificantly enhanced oscillator strength. FIG. 9 shows the thresholdenergy as a function of the concentration of SPCD in methanol. Thethreshold input pulse energy for spectral narrowing was observed todecrease with increased concentration of the SPCD solution. This furtherelucidates the dipolar order made easier by an increase in the numberdensity of the strongly dipolar excited species in the optical path.

[0063] For the organic dyes having large photoluminescence efficienciesbut smaller dipole moments, such as rhodamine 6G (R6G), the maximumefficiency that can be reached is slightly smaller. For rhodamine 6G themaximum efficiency is 30% at 10⁻³ mol/l concentration. For DCM, themaximum conversion efficiency is 25% at the concentration of 10⁻³ mol/l.The lowest threshold excitation pulse-energy for spectral narrowing inR6G is 1.5 μJ, and for DCM is 8 μJ.

[0064] Mirrorless solid-state lasing devices are also disclosed whichare constructed using the dipolar organic salts doped into solidmatrices of poly(methyl methacrylate) (PMMA). Spectrally narroweddirectional emission with typical bandwidth of ˜10 nm is achieved. FIG.10 illustrates the emission spectra of an SPCD doped PMMA solid film.The concentration of SPCD in PMMA was 5×10⁻³ mol/L. The evolution of thespectral linewidth of emission was measured as a function of theexcitation pulse energy for this device. The threshold excitation energywas about 4 μJ per pulse, beyond which the emission spectral widthdecreased from about 65 nm to about 10 nm. The threshold energy forspectrally narrowed emission was strongly dependent on the concentrationof SPCD in the PMMA matrix. The threshold was 25 μJ per pulse for aconcentration of 1×10⁻³ mol/L, and 4 μJ per pulse for a concentration of5×10⁻³ mol/L, both with a line excitation area of ˜5 mm². The emissioncharacteristics also strongly depended on the thickness of the films.

[0065] In accordance with the present invention, highly efficientmirrorless lasing devices using dipolar organic molecular materials atvery low threshold pump pulse energy have been constructed. The activematerial should possess either large dipole moment or highphotoluminescence efficiency. Significant narrowing of the spectrallinewidth was observed without using external mirrors. The thresholdenergy for spectral narrowing in the strongly dipolar molecular saltswas measured to be less than 1 μJ, and energy conversion efficiency ashigh as 40% was achieved. The output beam was strongly polarized andhighly directional with a low divergence. The extremely large Stokesshift of these materials ensures insignificant self-absorption of theemitted radiation. The low threshold and high energy conversionefficiency are attributed to the large excited state dipole moment inthis molecule. The dipole moments of the ground and excited states ofelectro-optic organic molecules have previously been calculated(Nonlinear Optical Properties of Organic Molecules and Crystals, D. S.Chemla and J. Zyss, Eds., Academic Press, New York (1987)). The excitedstate of the molecule is essentially formed via an electron transfermechanism and has a large dipole moment. In the presence of a strongoptical field these dipoles would form a highly correlated systemleading to efficient cooperative emission. The high conversion inmirrorless lasing in spite of low photoluminescence quantum efficiencyis a unique characteristic resulting from an exceptionally high degreeof dipolar order (or phasing) facilitated by the excited-state dipolemoments. Mirrorless laser-like emission with reasonably large conversionefficiencies was also achieved in organic dyes having highphotoluminescence efficiencies and moderate dipole moments, such asrhodamine 6G and DCM. The device can be used as an inexpensive frequencyshifter for short laser pulses. The generation of deep ultravioletradiation by frequency-doubling the output beam has also beendemonstrated.

[0066] The invention will be further described in connection with thefollowing examples, which are set forth for purposes of illustrationonly. Parts and percentages appearing in such examples are by weightunless otherwise stipulated.

EXAMPLES Example 1

[0067] In one embodiment of the present invention, the molecular saltstyrylpyridinium cyanine dye (SPCD) was used, the chemical formula ofwhich is given in FIG. 11. The cation in this molecule is a stilbazoliumchromophore, whereas the anion is methoxy-sulphonate. SPCD wassynthesized using the procedure similar to that reported in literature(S. R. Marder et al., Chem. Mater. 6, 1137 (1994)). The thresholdexcitation pulse energy for mirrorless laser-like emission in thisdipolar material was measured to be less than 1 μJ. Energy conversionefficiency as high as 40% was achieved at the concentration of 0.1mol/l. The emission peak was at 620 nm.

Example 2

[0068] In another embodiment of the present invention, the molecularsalt 4′-dimethylamino-N-methyl-4-stilbazolium tosylate (DAST) was used.The chemical formula of DAST is given in Table I. The cation in thismolecule is a stilbazolium chromophore, whereas the anion is toluenesulphonate. DAST was synthesized using the procedure similar to thatreported in literature (S. R. Marder et al., Chem. Mater. 6, 1137(1994)). The threshold excitation pulse energy for mirrorless laser-likeemission in DAST was less than 1 μJ, and energy conversion efficiency upto 35% was achieved. The emission peak was at 610 nm.

Example 3

[0069] In another embodiment of the present invention, the molecularsalt 4′-diethylamino-N-methyl-4-stilbazolium tosylate (DEST) was used.The chemical formula of DEST is given in Table I. The cation in thismolecule is a stilbazolium chromophore, whereas the anion is toluenesulphonate. DEST was synthesized using the procedure similar to thatreported in literature (S. R. Marder et al., Chem. Mater. 6, 1137(1994)). The threshold excitation pulse energy for mirrorless laser-likeemission in DEST was less than 1 μJ, and energy conversion efficiency upto 20% was achieved. The emission peak was at 617 nm.

Example 4

[0070] In another embodiment of the present invention, the molecularsalt 4′-dimethylamino-N-methyl-4-stilbazolium iodide (DASPI) was used.The chemical formula of (DASPI) is given in FIG. 11. The cation in thismolecule is a stilbazolium chromophore, whereas the anion is iodide.DASPI was synthesized using the procedure similar to that reported inliterature (S. R. Marder et al., Chem. Mater. 6, 1137 (1994)). Thethreshold excitation pulse energy for mirrorless laser-like emission inDEST was less than 1 μJ, and energy conversion efficiency up to 40% wasachieved. The emission peak was at 616 nm.

Example 5

[0071] In another embodiment of the present invention, the highlyefficient and widely used laser dye rhodamine 6G (R6G) was employed. R6Gwas purchased from Lambda Physik. The threshold excitation pulse energyfor mirrorless laser-like emission in R6G was 1.5 μJ, and energyconversion efficiency up to 30% was achieved at the concentration of10⁻³ mol/l. The emission peak was at 570 nm.

Example 6

[0072] In another embodiment of the present invention, the highlyefficient and widely used laser dye DCM was employed. DCM was purchasedfrom Lambda Physik. The threshold excitation pulse energy for mirrorlesslaser-like emission in DCM was 8 μJ, and energy conversion efficiency upto 25% was achieved at the concentration of 10⁻³ mol/l. The emissionpeak was at 641 nm. See FIG. 13.

[0073] Although only a few exemplary embodiments of this invention havebeen described in detail above, those skilled in the art will readilyappreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this invention. Accordingly, all such modifications areintended to be included within the scope of this invention as defined inthe following claims. It should further be noted that any patents,applications or publications referred to herein (and in the attached“References Cited” document) are incorporated by reference in theirentirety.

REFERENCES CITED U.S. PATENT DOCUMENTS

[0074] U.S. Pat. No. Issue Date Patentee Class 4,905,247 Feb. 1990Glessner et al. 372/55 5,237,582 Aug. 1993 Moses 372/53 5,825,790 Oct.1998 Lawandy 372/23 5,881,083 Mar. 1999 Diaz-Garcia et al. 372/395,912,257 Jun. 1999 Prasad et al. 514/356

OTHER PUBLICATIONS

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1. A highly efficient and low-threshold mirrorless laser (producinglaser-like emission without mirrors), comprising: a. at least onestrongly dipolar organic molecular salt having the following chemicalformula as the active media:

 where R and R′ are the same or different, and comprise a moietyselected from the group consisting of alkyl, substituted alkyl, benzyl,and substituted benzyl, and Y⁻ is an anion organic materials producinghighly efficient laser-like emission at low thresholds without externalmirrors in solution as active media; and b. a pump laser projecting theexcitation beam into the active media.